www.astrosociety.org/uitc No. 24 - Summer 1993 © 1993, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112. Black Holes by John Percy, University of Toronto Gravity is the midwife and the undertaker of the stars. It gathers clumps of gas and dust from the interstellar clouds, compresses them and, if they are sufficiently massive, ignites thermonuclear reactions in their cores. Then, for millions or billions of years, they produce energy, heat and pressure which can balance the inward pull of gravity. The star is stable, like the Sun. When the star's energy sources are finally exhausted, however, gravity shrinks the star unhindered. Stars like the Sun contract to become white dwarfs -- a million times denser than water, and supported by quantum forces between electrons. If the mass of the collapsing star is more than 1.44 solar masses, gravity overwhelms the quantum forces, and the star collapses further to become a neutron star, millions of times denser than a white dwarf, and supported by quantum forces between neutrons. The energy released in this collapse blows away the outer layers of the star, producing a supernova. If the mass of the collapsing star is more than three solar masses, however, no force can prevent it from collapsing completely to become a black hole. What is a black hole? Mini black holes How can you "see'' a Black Hole? Supermassive black holes Black holes and science fiction Activity #1: Shrinking Activity #2: A Scale Model of a Black Hole Black hole myths For Further Reading About Black Holes What is a black hole? A black hole is a region of space in which the pull of gravity is so strong that nothing can escape. It is a "hole'' in the sense that things can fall into it, but not get out. It is "black'' in the sense that not even light can escape. Another way to say it, is that a black hole is an object for which the escape velocity (the velocity required to break free from an object) is greater than the speed of light -- the ultimate "speed limit'' in the universe. In 1783, British amateur astronomer, Rev. John Mitchell, realized that Newton's laws of gravity and motion implied that the more massive an object, the greater the escape velocity. If you could somehow make something 500 times bigger than the Sun, but with the same density, he reasoned, even light couldn't move fast enough to escape from it and it would never be seen. But it took Einstein's general theory of relativity, the modern theory of gravity, for astronomers and physicists to understand the true nature and characteristics of black holes. The boundary of a black hole is called the event horizon, because any event which takes place within is forever hidden to anyone watching from outside. Astronomer Karl Schwarzschild showed that the radius of the event horizon in kilometers is 3 times its mass expressed in units of solar masses; this radius is called the Schwarzschild radius. The event horizon is the one-way filter in the black hole: anything can enter, but nothing can leave. A black hole is a very simple object: it has only three properties mass, spin and electrical charge. Because of the way in which black holes form, their electrical charge is probably zero, which makes them simpler yet. The form of the matter in a black hole is not known, partly because it is hidden from the outer universe, and partly because the matter would, in theory, continue to collapse until it had a radius of zero, a point mathematicians call a singularity, of infinite density -- something with which we have no experience here on Earth. Black holes are theorized to come in three different sizes: small ("mini''), medium, and large ("supermassive''). There is good evidence that medium-sized black holes form as the corpses of massive stars which collapse at the end of their lives, and that supermassive black holes exist in the cores of many galaxies -- perhaps including our own. Mini Black Holes A black hole with a mass less than three solar masses would not form on its own; its gravity is too weak to cause it to completely collapse in on itself. Enormous outside pressure would be necessary to create a "mini- black hole.'' In 1971, astrophysicist Stephen Hawking theorized that, in the dense turbulence of the Big Bang from which the universe emerged, such enormous outside pressures existed and many mini-black holes formed. These would be as massive as mountains, but as small as the protons of which atoms are made. They would have another bizarre property: as a result of the laws of quantum mechanics which govern very small particles in the universe, they would spontaneously radiate energy and, after billions of years, eventually evaporate in a final violent explosion. Thus, mini-black holes may not be entirely "black" -- an intriguing possibility. No observational evidence of mini-black holes exists but, in principle, there could be such objects scattered throughout the universe, perhaps even near our solar system. Black Holes How can you "see'' a black hole? You might wonder how a black hole could be found if nothing including light can escape from it. Black holes have mass, which causes a gravitational force, which effects objects near them. This gravitational force would be very strong near the black hole, and could have noticeable effects on its environment. Material falling into the black hole would gain energy from the gravitational field, and would be crushed and heated as it tried to squeeze into the black hole's tiny throat, causing it to emit x-rays. The first example of a black hole was discovered by just such a gravitational effect on a companion star. An artist's conception of the Cygnus X-1 system. HDE 226868 is a massive blue supergiant star; its companion is believed to be a black hole, surrounded by an accretion disc of gases from HDE 226868 which are spiraling into the black hole. The star and the black hole are in orbit around each other. The black hole's existence was deduced from the orbital motion of the star, and from the X-rays produced by the gas in the accretion disc which is heated as it falls toward the black hole. (Courtesy William J. Kaufmann III, Universe, W.H. Freeman & Company, 1991. Used with permission.) Cygnus X-1 was the name given to a source of x-rays in the constellation Cygnus, discovered in 1962 with a primitive x-ray telescope flown on a rocket. By 1971, the location of the x-ray source in the sky had been measured more precisely, using rocket and satellite observations. A key breakthrough came in March 1971, when a new source of radio waves was discovered in Cygnus, near the position of the x-ray source. The radio signal varied at the exact same time when the strength of the x-rays changed strong evidence that the radio and x-ray sources were the same object. A faint star called HDE 226868 appears at the position of this radio source. Astronomers studying the light of HDE 226868 have found two important facts: (1) HDE 226868 is a blue supergiant star -- a massive, normal star near the end of its life; and (2) the star is orbiting another massive object in a 5.6-day orbit. From the gravitational force needed to keep HDE 226868 in orbit, the mass of the companion can be determined -- it is about 10 solar masses. But there is no sign of any visible light from the companion -- and something in the object produces x-rays. The explanation or "model'' which best fits these facts is that the companion is a black hole of about 10 solar masses -- the corpse of a massive star which was once the companion of HDE 226868. The x-rays are produced as gas from the atmosphere of the blue supergiant star falls into the collapsed object and is heated. The collapsed object cannot be a white dwarf or neutron star, because these objects can't have masses greater than 1.44 and three solar masses, respectively. We may never be able to "prove'' this theory of Cygnus X-1 by "seeing'' the black hole, but the circumstantial evidence is strong. Three other objects -- LMC X-3 in the Large Magellanic Cloud galaxy, and A0620-00 and V404 Cygni in our galaxy -- are also believed to have black holes as one of their components. Supermassive black holes A quarter of a century ago, astronomers discovered distant objects rare, distant objects which were producing extraordinarily large power in an extraordinarily small volume -- the power of a trillion Suns in a volume not much larger than the solar system. They called these objects quasi-stellar radio sources -- quasars, for short - - because they looked like stars, and produced large amounts of radio waves as well as light. Astronomers also realized that, although quasars were rare, there were many other objects -- apparently galaxies of stars - - which showed less extreme versions of the same phenomenon: very large power from a very small volume. These objects shared another remarkable property: jets of high-energy particles emitted from their cores. These properties were so difficult to explain, using the physical knowledge of the time, that some astronomers even questioned whether that knowledge was correct! In the years since, astronomers realized that there is an explanation for these active galactic nuclei which is consistent with observations and with theory -- even though this explanation boggles the mind: at the core of these galaxies is a supermassive black hole, with the mass of millions or billions of Suns.